DENVER—Marijuana driving limits won unanimous approval in a Colorado legislative committee Tuesday, after lawmakers from both parties said that if voters want pot treated like alcohol, maximum blood levels for drivers are imperative.

The House Judiciary Committee voted 11-0 for the blood standard to help authorities determine whether a driver is too stoned to be behind the wheel.

After testimony from law enforcement and a toxicologist, House members agreed that drivers should be considered impaired if their blood contains more than 5 nanograms of THC per milliliter. THC is the psychoactive ingredient in marijuana.

Democratic Rep. Rhonda Fields, sponsor of the bill, compared it to early attempts to use blood tests to convict drunk drivers.

"It is time for us to have a shift in our mindset" now that marijuana is legal in Colorado, Fields argued.

"There was a time when people thought it was OK to drink alcohol and drive."

No one argued Tuesday that stoned driving is safe. But driving-high proposals have failed three times in Colorado because of concerns that blood tests aren't a fair way to tell whether someone is too high to get behind the wheel. Marijuana activists argued that blood THC limits don't accurately indicate impairment, and that Colorado should stick with its current reliance on officer observation that someone is stoned.

"I'm not impaired. I'm not a drug abuser. I'm not an addict. I'm a responsible member of society," testified Teri Robnett, a marijuana user who suffers from fibromyalgia.

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Robnett argued that sick marijuana patients have elevated THC blood levels and could be unfairly prosecuted even if they're sober to drive. She also argued that marijuana users have no easy way to self-test to find out if their blood levels are within legal parameters before driving, and that marijuana users know when they're impaired.

"It's not the liquid courage alcohol is," Robnett said.

Prosecutors pointed out that under the bill, chronic pot users could argue in court that they were unimpaired despite a THC level above 5 nanograms. The same is not true in alcohol DUI cases, where defendants who exceed legal limits can't argue they were OK to drive despite their blood-alcohol level.

An assistant Colorado attorney general testified that even if marijuana is legal, public safety demands a driving limit.

"There is no constitutional right to jeopardize public safety, and that's what this bill is about," Matt Durkin argued.

The driving-stoned bill awaits one more committee vote before it heads to the House floor. The greater test is in the state Senate, where marijuana blood limits have failed three times before. Passage is considered more likely this year, and Gov. John Hickenlooper supports the blood limit.

"We were told by the people of Colorado that we have to treat marijuana like alcohol," concluded Rep. Pete Lee, D-Colorado Springs.